In the field of X-ray detection it is well-known to employ so-called intensifying screens to increase the radiation available for detection purposes. Such screens contain an X-ray luminescent material which is selected to emit a relatively large number of light photons for each X-ray photon striking the material. This effectively amplifies the X-rays to be detected since both the X-rays themselves and light emitted by X-ray-induced emission from the luminescent material are available for detection on film or other detection mediums or devices, such as arrays of light-sensitive electronic sensors. The primary incentive to use such intensifying screens in medical applications is to reduce the amount of X-ray radiation which is required to produce a given exposure, thereby reducing the radiation risk to which a patient or operator is exposed.
It is known that such intensifying screens, while increasing the amount of radiation available for detection, also have the effect of reducing the sharpness of the resultant image. In general, image distortion in luminescent screens or structures is caused by the diffusion of light within the luminescent material which causes a blurring of the image with consequent loss of definition and contrast. This diffusion of light is brought about by two fundamental physical processes. First, as the ionizing radiation is converted into light, the direction of emission of light is random so that it is emitted approximately equally in all directions. The second effect is that the high energy radiation is penetrating, the degree of penetration being dependent upon the energy of the impinging radiation and the nature of the material being penetrated. The higher the energy, the deeper the penetration. A lower density material will also lead to a deeper penetration.
Thus, it is seen that as visible light is generated along a path through the screen and normal to its surface, light will be radiating in all directions. Some of the light radiated at an angle off the normal to the surface of the screen will reach the film or other detecting means and result in a diffuse image.
As a result, the design of such intensifying screens has involved a trade-off between screens of large thickness, which result in increased luminescent radiation for a given X-ray level, but which also produce decreased image sharpness, and screens of less thickness, which result in improved image sharpness relative to the thicker screens, but which also require more X-ray radiation to produce acceptable film images, thereby increasing the X-ray dosage to which the patient must be exposed. In practice, the thicker or high speed screens are utilized in those applications which do not require maximum image sharpness, thereby reducing the patient exposure to X-rays, while medium speed and slow speed screens are utilized when increased image resolution is required. These latter screens employ thinner phosphor layers and may incorporate dyes to minimize transverse propagation of light by attenuating such rays more than a normal ray which travels a shorter path. In general, detail or slow speed screens require approximately 8 times the X-ray dosage of high speed screens.
Several patents have proposed solutions to the problem of reducing the amount of scattered luminescent radiation which reaches the film or other detector from such screens. These patents have suggested a cellularized or pixelized approach to the construction of such screens, the structure generally consisting of volumes of luminescent material separated by wall members. The wall members are disposed generally parallel to the direction of X-ray travel and their purpose is to reflect light emitted by the luminescent material and thereby prevent scattered light from reaching the detection means.
One such approach is taught in U.S. Pat. No. 3,041,456, in which a rectangular body of plastic having a luminescent phosphor dispersed therein is sliced into thin slices which are then coated on one or both sides with a reflective material. These coated slices are then bonded back together and sliced again in a direction transverse to that of the first slicing. These coating and bonding operations are repeated to produce a double laminated body from which screens of the desired thickness may be obtained. The approach of this patent, while being theoretically attractive, presents significant problems in manufacturing because of the requirement to repeatedly handle and align extremely small pieces of the phosphor without damage or contamination.
An alternative approach is suggested in U.S. Pat. No. 3,643,092. The structures proposed there employ adjacent walls having a corrugated member disposed therebetween so as to form a plurality of chambers extending in the direction of X-ray travel. At least a portion of each of these chambers is filled with a luminescent phosphor which reacts to X-ray radiation in the manner described above to produce light. The chamber structure is such that the walls thereof, formed by the planar wall members and the corrugated member, confine and/or reflect emitted light so as to limit the amount of scattered radiation reaching the detection means. The structures proposed in this patent, like that of U.S. Pat. No. 3,041,456, are attractive in theory, but present problems in fabrication because of the requirement to handle the small and fragile components.
Other literature has suggested that chemical etching or milling be employed to produce grooves in a phosphor material, the grooves then being filled or plated with a highly reflective material to form light reflecting walls. However, this type of etching or milling produces surfaces which are relatively rough, so that even though subsequently plated or coated, they do not provide a good reflective surface. Such relatively rough surfaces have the effect of producing multiple reflections so that much of the light is lost through severe scattering.
An additional disadvantage of such chemical milling or etching is that the walls produced must be at least 0.003-0.010 inches thick in order to provide sufficient strength for handling of the structure. Walls of this thickness are discernable and result in corresponding lines appearing in the image on the film, thereby reducing the resolution. Additionally, walls of this thickness reduce the amount of available phosphor by a corresponding amount, thus reducing the light output from the structure. Further, these structures have the disadvantage that the circumference of walls are continuous and rigid so that when the phosphor cures after being poured or impregnated into the cells, shrinkage or expansion may occur. This often results in fracturing of the phosphor with a resultant poor light transmission due to the separated interface at the fracture.
U.S. Pat. No. 3,936,645 discloses a cellularized luminescent structure which is fabricated by utilizing a laser to cut narrow slots in the luminescent material in both the X and Y directions. The slots are then filled with material which is opaque to either light or radiation or both. There is no disclosure of utilizing a phosphor material, however, to fill in the slots to create cellularized ("pixelized") phosphors separated by slots as narrow as 0.5 microns in width.
U.S. Pat. No. 5,153,438 discloses a structured scintillator material wherein the gaps between the individual scintillator elements are preferably filled in with a reflective material such as titanium dioxide, magnesium oxide, etc., in order to maximize the portion of light within each element that is collected by its associated photosensitive cell. In this patent, the individual elements are formed by preferential deposition of the phosphor over structures existing on the surface of the substrate. Again, as with U.S. Pat. No. 3,936,645, there is no disclosure in the 3 438 patent of utilizing a phosphor material to fill in slots to create pixelized phosphors separated by slots as narrow as 0.5 microns in width.